This invention relates to semiconductor circuits and more particularly to static driver circuits of the type useful as outputs for semiconductor memory devices or the like.
In static semiconductor memory devices of the RAM, ROM or EPROM type, a power-down mode of operation is often used. A chip-select power-down mode is now commonly used. Usually, a gating transistor is connected in series with each key inverter that draws direct current, with the gates of all gating transistors connected to a chip-select CS signal so that these transistors will be turned off when the chip is not selected or CS is not active. In the typical push-pull output buffer circuit used in this type of device, when a single +5 V supply is employed, a problem arises because the output level for a logic 1 is a two Vt drop from the Vcc supply due the threshold voltages of the output transistor and its gating transistor. This output level is often marginal for achieving a 2.4 V TTL level specification under a source current of 400 microamp. In order to minimize the Vt drop, the output and gating transistors are constructed as "natural" transistors with a Vt of typically 0 to +0.3 V. Even at this low Vt, the output "1" level is still marginal because of increase net Vt due to body effect.
It is therefore the principal object of this invention to provide an improved static driver circuit, particularly one that is adapted for low power dissipation and has a low voltage drop. Another object is to provide a push-pull MOS output circuit which has high speed and is adapted for a power-down mode of operation with low power dissipation, yet produces a full logic level output voltage.